Method of production of 2,4-dihydroxybutyric acid

ABSTRACT

A method of producing 2,4-dihydroxybutyric acid (2,4-DHB) by a synthetic pathway that includes transforming malate into 4-phospho-malate using a malate kinase, then transforming 4-phospho-malate into malate-4-semialdehyde using a malate semialdehyde dehydrogenase, and then transforming malate-4-semialdehyde into 2,4-DHB using a DHB dehydrogenase.

This is a continuation of application Ser. No. 13/882,372 filed Jun. 20, 2013, which is a National Stage Application of PCT/IB2011/002870 filed Oct. 27, 2011, and claims the benefit of International Application Nos. PCT/IB2010/003153 filed Oct. 28, 2010 and PCT/IB2011/001559 filed May 23, 2011. The entire disclosures of the prior applications are hereby incorporated by reference in their entirety.

The present invention relates to a novel method of production of 2,4-dihydroxybutyric acid from malate by the implementation of a synthetic pathway that comprises enzymes having malate kinase, malate semialdehyde dehydrogenase, and 2,4-dihydroxybutyrate dehydrogenase activity, respectively.

The carboxylic acids cited within the present application are equally named under their salt (e.g. 2,4-dihydroxyburyrate) or acid forms (e.g. 2,4-dihydroxybutyric acid).

2,4-dihydroxybutyric acid (equally 2,4-DHB or DHB) is a compound of considerable economic interest. DHB can be readily converted into α-hydroxy-γ-butyrolactone in aqueous media by adjusting the appropriate pH. α-hydroxy-γ-butyrolactone is a prominent precursor for the production of the methionine substitute 2-hydroxy-4-(methylthio)-butyrate (HMTB) (Deck et al., 2008) which has a large market in animal nutrition. At present, α-hydroxy-γ-butyrolactone is derived from γ-butyrolactone by a multi-stage process that implies halogenation of the γ-butyrolactone in position α, and subsequent substitution of the halogen atom by a hydroxyl group in alkaline medium (Deck et al., 2008).

From growing oil prices the need for the production of DHB from renewable resources arises. Microorganisms are capable of transforming biomass-derived raw material, e.g. sugars or organic acids, into a large variety of different chemical compounds (Werpy & Petersen, 2004). With the growing body of biochemical and genomic information it is possible to modify microorganisms such that they overproduce naturally occurring metabolic intermediates with high yield and productivity (Bailey, 1991). Optimization of production microorganisms often requires rational engineering of metabolic networks which ensures, among others, overexpression of enzymes required for the biosynthesis of the metabolite of interest, and alleviation of product feedback inhibition. Another possibility is the implementation of novel enzymatic systems that catalyze the production of a metabolite of interest.

Metabolic engineering approaches and enzymatic catalyses require detailed knowledge on the biochemistry and regulation of the metabolic pathway leading to the metabolite of interest. In the case of DHB production, this information is not available. Only few studies report the occurrence of DHB in patients with succinic semialdehyde dehydrogenase deficiency (Shinka et al., 2002) without, however, identifying enzymatic reactions implicated in DHB production. The zymotic or enzymatic production of DHB, therefore, requires (i) the identification of a thermodynamically feasible pathway which transforms an accessible precursor into DHB, (ii) the identification or construction of enzymes that are capable to catalyze individual reaction steps in the pathway and (iii) the functional expression of the pathway enzymes in an appropriate production organism.

Accordingly, one object of the present invention is a method of producing 2,4-DHB comprising a first step of transforming malate in 4-phospho-malate using a malate kinase, a second step of transforming 4-phospho-malate in malate-4-semialdehyde using a malate semialdehyde dehydrogenase, a third step of transforming malate-4-semialdehyde in 2,4-DHB using a DHB dehydrogenase.

In the first reaction (see FIG. 1 (i)), malate (1) is converted into 4-phospho-malate (2) by the action of an enzyme which possesses malate kinase activity (A). In the second reaction (B), 4-phospho-malate is converted into malate-4-semialdehyde (3) by the action of an enzyme which possesses malate semialdehyde dehydrogenase activity. More precisely, reaction (B) is catalysed by an enzyme bearing dephosphorylating 4-phospho-malate reductase activity in the biosynthetic sense of the pathway. In the third reaction (C), malate-4-semialdehyde is converted into DHB (4) by the action of an enzyme which possesses DHB dehydrogenase activity. More precisely, reaction (C) is catalysed by an enzyme bearing malate-4-semialdehyde reductase activity in the biosynthetic sense of the pathway.

None of the above cited enzymes and intermediary products have been so far neither described nor identified in living cells. As such malate kinase, malate semialdehyde dehydrogenase, DHB dehydrogenase and 4-phospho-malate are further objects of the invention.

Within another aspect of the invention, the first step of the method of producing 2,4-DHB involves a malate kinase that is characterized in that it transforms malate into 4-phospho-malate. Said enzyme is obtainable by at least one mutation of an enzyme, said mutation(s) improving the activity and/or substrate affinity of the mutated enzyme for malate.

Within the present invention, the expression “improve the activity and/or substrate affinity” means that the enzyme before mutation, was either

-   -   unable to use the substrate (malate, 4-phospho-malate or         malate-4-semialdehyde), and/or     -   synthesized the product of the reaction (4-phospho-malate or         malate-4-semialdehyde or DHB) at a maximum specific rate at         least three times lower, and/or     -   had an affinity for malate, 4-phospho-malate or         malate-4-semialdehyde that was at least three times lower,         and/or     -   had an affinity for the natural substrate (aspartate,         4-phospho-asparte, aspartate-4-semialdehyde) which was at least         3 times higher.

Within another of its aspects the invention deals with the use of a malate kinase to transform malate into 4-phospho-malate.

The malate kinase activity can be measured by the enzymatic test described in example 1 (see “Enzymatic assay”).

According to another aspect of the invention, the malate kinase can be obtained by mutation of an aspartate kinase.

FIG. 2 shows the alignment of amino acid sequences of aspartate kinases of different biological origin. All references to the position of amino acids are made based on the amino acid sequence of aspartate kinase encoded by the LysC gene of E. coli (represented by SEQ ID NO: 4). The relative positions of corresponding conserved regions in the other aspartate kinases from different organisms can easily be found by the man skilled in the art by simple sequence alignment as represented in FIG. 2 with enzymes listed below:

-   -   AKIII—aspartate kinase III of E. Coli (SEQ ID NO: 4),     -   AKI (SEQ ID NO: 87) aspartate kinase I of E. coli,     -   AKII (SEQ ID NO: 88)—aspartate kinase II of E. coli,     -   MJ—Methanococcus jannaschii (SEQ ID NO: 89),     -   TT—Thermus thermophilus (SEQ ID NO: 90),     -   CG—Corynebacterium glutamicum (SEQ ID NO: 91),     -   AT—Arabidopsis thaliana (SEQ ID NO: 92),     -   SC—Saccharomyces cerevisiae. (SEQ ID NO: 93).

Said alignment can be done with the ClustalW2 software. For example, the E119 residue of the aspartate kinase represented by SEQ ID NO: 4 corresponds to the E207 residue of the aspartate kinase of A. thaliana (SEQ ID NO: 50) or to the E147 residue of the aspartate kinase of S. cerevisiae (SEQ ID NO: 51).

The mutated aspartate kinase according to the invention comprises at least one mutation, when compared to the wild type enzyme, in at least one of the following positions: S39, T45, V115, E119, F184 and/or S201, wherein the naturally occurring amino acid in said positions is replaced by anyone of the other 19 naturally existing proteinogenic amino acids, that is by either alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, or valine.

In a non-exclusive example, the construction of a malate kinase by site directed mutagenesis is demonstrated using the aspartate kinase Lys C of Escherichia coli as the template. According to one aspect of the invention, the substrate specificity of LysC was changed towards malate by replacing the glutamic acid in position 119 by either asparagine, glutamine, cysteine, proline, serine, threonine, valine or glycine.

Within a further aspect of the invention, the malate kinase is represented by SEQ ID NO: 9, and more specifically by SEQ ID NO:12, SEQ ID NO:14, SEQ ID NO:16, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24 or SEQ ID NO:26.

Aspartate kinases are typically inhibited by either methionine, threonine or lysine. Therefore, malate kinases that were constructed by random or site directed mutagenesis of aspartate kinases may also be inhibited by said amino acids. In a further aspect of the invention, the inhibition of malate kinase by methionine, lysine or threonine is alleviated by mutagenesis of the malate kinase.

In a specific aspect of the invention, the above described mutated LysC (malate kinase) is rendered insensitive to lysine inhibition by mutation of at least one the following amino acids E250, M318, S321, V339, S338, F324, L325, V339, S345, E346, D340, T344 and/or T352 (see example 3).

The present invention also encompasses such modified enzymes and more particularly those represented by SEQ ID NO: 39, SEQ ID NO:41, SEQ ID NO:43 or SEQ ID NO:45.

Within a still further aspect, the second step of the method of producing 2,4-DHB according to the invention involves a malate semialdehyde dehydrogenase characterized in that it transforms 4-phospho-malate into malate-4-semialdehyde, said enzyme bearing a dephosphorylating 4-phospho-malate reductase activity in the biosynthetic sense of the pathway.

The malate semialdehyde dehydrogenase activity can be measured by the enzymatic test described in example 4 (see “Enzymatic assay”).

This enzyme is obtainable by at least one mutation of an enzyme, said mutation(s) improving the activity and/or substrate affinity of the mutated enzyme for 4-phospho-malate.

According to another aspect, the malate semialdehyde dehydrogenase of the invention can be obtained by mutation of an enzyme having reported semialdehyde dehydrogenase activity, more specifically having dephosphorylating activity in the reductive sense of the reaction, more specifically acting on organic molecules that consist of 3, 4, or 5 carbon molecules. In a specific aspect of the invention said malate semialdehyde dehydrogenase is obtained by mutation of an aspartate semialdehyde dehydrogenase.

The aspartate semialdehyde dehydrogenase, Asd of E. coli and Hom2 of Saccharomyces cerevisiae naturally exhibit dehydrogenase activity on 4-phospho-malate 2.

According to another aspect of the invention, the malate semialdehyde dehydrogenase can be improved by the mutation of aspartate semialdehyde dehydrogenase.

FIG. 3 shows the alignment of amino acid sequences of aspartate semialdehyde dehydrogenases of different biological origin. All references to amino acids are made based on the aspartate semialdehyde dehydrogenase encoded by the Asd gene of E. coli (as represented by SEQ ID NO: 20). The relative positions of corresponding conserved regions in the other aspartate semialdehyde dehydrogenases from different organisms can easily be found by the man skilled in the art by simple sequence alignment as represented in FIG. 4 with enzymes listed below:

-   -   EC—E. Coli (SEQ ID NO: 49),     -   MJ—Methanococcus jannaschii (SEQ ID NO: 94),     -   TT—Thermus thermophilus (SEQ ID NO: 95),     -   BS—Bacillus subtilis (SEQ ID NO: 96),     -   CG—Corynebacterium glutamicum (SEQ ID NO: 97),     -   AT—Arabidopsis thaliana (SEQ ID NO: 98),     -   SC—Saccharomyces cerevisiae. (SEQ ID NO: 99)

Said alignment can easily be done using the ClustalW2 software.

The construction of enzymes having improved malate semialdehyde dehydrogenase activity can be done as follows.

The malate semialdehyde dehydrogenase according to the invention corresponds in a specific aspect to an aspartate semialdehyde dehydrogenase comprising at least one mutation when compared to the wild type enzyme in at least one of the positions T136, Q162, 1230, E241 and/or H274 wherein the naturally occurring amino acid in said positions is replaced by anyone of the other 19 naturally existing proteinogenic amino acids, that is by either alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, or valine.

As demonstrated in Example 5, site-directed mutagenesis of asd from E. coli can improve activity and substrate affinity of the mutated enzyme for 4-phospho-malate, at the same time diminishing the preference of the enzyme for its natural substrate 4-phospho-aspartate.

In order to improve the activity of Asd on 4-phospho-malate, and according to one aspect of the invention, E241 was replaced by a glutamine, alanine, cysteine, glycine, histidine, isoleucine or methionine residue by site directed mutagenesis (Example 5).

Within a further aspect of the invention, the malate semialdehyde dehydrogenase is represented by SEQ ID NO: 68 and more specifically by SEQ ID NO:54, SEQ ID NO:56, SEQ ID NO:58, SEQ ID NO:60, SEQ ID NO:62, SEQ ID NO:64 or SEQ ID NO:66.

Within another of its aspect the invention deals with the use of a malate semialdehyde dehydrogenase to transform 4-phospho-malate in malate-4-semialdehyde.

Within another aspect, the third step of the method of producing 2,4-DHB according to the invention involves a DHB dehydrogenase characterized in that it transforms malate-4-semialdehyde into 2,4-DHB, said enzyme bearing malate-4-semialdehyde reductase activity in the biosynthetic sense of the pathway.

Candidate DHB dehydrogenase enzymes that potentially already possess DHB dehydrogenase activity can be chosen from the class of beta-hydroxyacid dehydrogenases that act on C3, C4, or C5 compounds.

According to a still further aspect of the invention, said DHB dehydrogenase enzymes can be structurally and mechanistically related to β-hydroxyacid dehydrogenases such as tartronate semialdehyde reductases, succinate semialdehyde reductases, malonate semialdehyde reductases, methylbutyraldehyde reductases, zinc-type alcohol dehydrogenases, L-threonine-3-dehydrogenases, or homoserine reductases.

The present invention also deals with the use of a methylbutyraldehyde reductase or of a succinic semialdehyde reductase to transform malate-4-semialdehyde in 2,4-DHB. In specific embodiments, said methylbutyraldehyde reductase is represented by SEQ ID NO: 74 and said succinic semialdehyde reductase is represented by SEQ ID NO: 76. The DHB dehydrogenase activity can be measured by the enzymatic test described in example 6 (see “Enzymatic assay”).

The affinity of DHB dehydrogenase for malate-4-semi aldehyde can be increased by at least one mutation of an enzyme, said mutation(s) increasing the activity and/or substrate affinity of the mutated enzyme for malate-4-semialdehyde, and/or reducing the activity or affinity for its natural substrate by at least factor 2.

The DHB dehydrogenase according to the invention corresponds in a specific aspect to M. sedula succinic semialdehyde reductase (SEQ ID No76) comprising at least one mutation when compared to the wild type enzyme in at least one of the positions S40, N43, H39 T49, F85, Q108, L281 and N305 wherein the naturally occurring amino acid in said positions is replaced by anyone of the other 19 naturally existing proteinogenic amino acids, that is by either alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, or valine.

As demonstrated in a non-exclusive example, the affinity of M sedula succinic semialdehyde reductase for (L)-malate-4-semialdehyde was increased by introducing the double mutation H39R N43H by site-directed mutagenesis, as represented by SEQ ID NO: 81. Simple mutants H39R (SEQ ID NO: 225) and N43H (SEQ ID NO: 227) are also encompassed by the present invention (Example 7).

DHB dehydrogenase can be used to transform malate-4-semialdehyde into 2,4-DHB, which constitutes a further aspect of the invention.

The nucleic acid sequence of genes can be adapted to the codon usage of the host organism thereby increasing the production of the heterogeously expressed proteins. This constitutes a further aspect of the invention.

The synthesis of a synthetic gene coding for M. sedula succinic semialdehyde reductase H39R N43H whose nucleotide sequence was optimized for the expression of said enzyme in E. Coli as represented by SEQ ID NO:228 is a further aspect of the invention.

In a still further aspect, the present invention also deals with nucleic acids, and more particularly with isolated nucleic acid sequences encoding a malate kinase as above described.

In another aspect, said nucleic acid is represented by SEQ ID NO: 13, SEQ ID NO:15, SEQ ID NO:17, SEQ ID NO:19, SEQ ID NO:21, SEQ ID NO:23, SEQ ID NO:25, SEQ ID NO:27, SEQ ID NO:38, SEQ ID NO:40, SEQ ID NO:42 or SEQ ID NO:44.

In a still further aspect, the present invention also deals with isolated nucleic acid sequences encoding a malate semialdehyde dehydrogenase as above described.

More specifically, said nucleic acid is preferentially represented by SEQ ID NO: 55, SEQ ID NO:57, SEQ ID NO:59, SEQ ID NO:61, SEQ ID NO:63, SEQ ID NO:65 or SEQ ID NO:67.

In a still further aspect, the present invention also deals with isolated nucleic acid sequences encoding a DHB dehydrogenase as above described.

In another aspect, said nucleic acid is represented by SEQ ID NO: 73 or SEQ ID NO: 75, SEQ ID NO: 224, SEQ ID NO: 226 or SEQ ID NO:82

In accordance with this invention, a “nucleic acid sequence” refers to a DNA or RNA molecule in single or double stranded form, preferably a DNA molecule. An “isolated DNA”, as used herein, refers to a DNA which is not naturally-occurring or no longer in the natural environment wherein it was originally present, e.g., a DNA coding sequence associated with other regulatory elements in a chimeric gene, a DNA transferred into another host cell, or an artificial, synthetically-made DNA sequence having a different nucleotide sequence compared to any naturally-occurring DNA sequence.

The present invention also relates to a chimeric gene comprising, functionally linked to one another, at least one promoter which is functional in a host organism, a polynucleotide encoding anyone of the malate kinase, malate semialdehyde dehydrogenase or DHB dehydrogenase according to the invention, and a terminator element that is functional in the same host organism. The various elements which a chimeric gene may contain are, firstly, elements regulating transcription, translation and maturation of proteins, such as a promoter, a sequence encoding a signal peptide or a transit peptide, or a terminator element constituting a polyadenylation signal and, secondly, a polynucleotide encoding a protein. The expression “functionally linked to one another” means that said elements of the chimeric gene are linked to one another in such a way that the function of one of these elements is affected by that of another. By way of example, a promoter is functionally linked to a coding sequence when it is capable of affecting the expression of said coding sequence. The construction of the chimeric gene according to the invention and the assembly of its various elements can be carried out using techniques well known to those skilled in the art. The choice of the regulatory elements constituting the chimeric gene depends essentially on the host organism in which they must function, and those skilled in the art are capable of selecting regulatory elements which are functional in a given host organism. The term “functional” is intended to mean capable of functioning in a given host organism.

The promoters which the chimeric gene according to the invention may contain are either constitutive or inducible. By way of example, the promoters used for expression in bacteria may be chosen from the promoters mentioned below. For expression in Escherichia coli mention may be made of the lac, trp, Ipp, phoA, recA, araBAD, prou, cst-I, tetA, cadA, nar, tac, trc, lpp-lac, Psyn, cspA, PL, PL-9G-50, PR-PL, T7, [lambda]PL-PT7, T3-lac, T5-lac, T4 gene 32, nprM-lac, VHb and the protein A promoters or else the Ptrp promoter (WO 99/64607). For expression in Gram-positive bacteria such as Corynebacteria or Streptomyces, mention may be made of the PtipA or PS1 and PS2 (FR91/09870) promoters or those described in application EP0629699A2. For expression in yeasts and fungi, mention may be made of the K. lactis PLAC4 promoters or the K. lactis Ppgk promoter (patent application FR 91/05294), the Trichoderma tef1 or cbh1 promoter (WO 94/04673), the Penicillium his, csl or apf promoter (WO 00/68401) and the Aspergillus gla promoter.

According to the invention, the chimeric gene may also comprise other regulatory sequences, which are located between the promoter and the coding sequence, such as transcription activators (enhancers).

As such, the chimeric gene of the invention comprises in a specific embodiment at least, in the direction of transcription, functionally linked, a promoter regulatory sequence which is functional in a host organism, a nucleic acid sequence encoding the malate kinase of the malate semialdehyde dehydrogenase of the invention and a terminator regulatory sequence which is functional in said host organism

The present invention also relates to a cloning and/or expression vector comprising a chimeric gene according to the invention or a nucleic acid sequence of the invention. The vector according to the invention is of use for transforming a host organism and expressing in this organism anyone of the malate kinase, malate semialdehyde dehydrogenase and/or DHB dehydrognase. This vector may be a plasmid, a cosmid, a bacteriophage or a virus. Preferentially, the transformation vector according to the invention is a plasmid. Generally, the main qualities of this vector should be an ability to maintain itself and to self-replicate in the cells of the host organism, in particular by virtue of the presence of an origin of replication, and to express anyone of the malate kinase, malate semialdehyde dehydrogenase and/or DHB dehydrognase therein. For the purpose of stable transformation of a host organism, the vector may also integrate into the genome. The choice of such a vector, and also the techniques of insertion of the chimeric gene according to the invention into this vector and are part of the general knowledge of those skilled in the art. Advantageously, the vector used in the present invention also contains, in addition to the chimeric gene according to the invention, a chimeric gene encoding a selectable marker. This selectable marker makes it possible to select the host organisms which are effectively transformed, i.e. those which incorporated the vector. According to a particular embodiment of the invention, the host organism to be transformed is a bacterium, a yeast, a fungus. Among the selectable markers which can be used, mention may be made of markers containing genes for resistance to antibiotics, such as, for example, the hygromycin phosphotransferase gene. Other markers may be genes to complement an auxotrophy, such as the pyrA, pyrB, pyrG, pyr4, arg4, argB and trpC genes, the molybdopterin synthase gene or that of acetamidase. Mention may also be made of genes encoding readily identifiable enzymes such as the GUS enzyme, or genes encoding pigments or enzymes regulating the production of pigments in the transformed cells. Such selectable marker genes are in particular described in patent applications WO 91/02071, WO 95/06128, WO 96/38567 and WO 97/04103.

The present invention also relates to transformed host organisms containing at least one chimeric gene according to the invention, either integrated into their genome or carried on an extrachromosomal genetic element, for example a plasmid. In a more specific aspect of the invention, the transformed host organism comprises a nucleic acid of the invention encoding a malate kinase or a chimeric gene comprising a nucleic acid encoding a malate kinase or an expression vector comprising a nucleic acid encoding a malate kinase, and/or a nucleic acid encoding a malate semialdehyde dehydrogenase, or a chimeric gene comprising a nucleic acid encoding a malate semialdehyde dehydrogenase or an expression vector comprising a nucleic acid encoding a malate semialdehyde dehydrogenase, and/or a nucleic acid encoding a DHB dehydrogenase, a chimeric gene comprising a nucleic acid encoding a DHB dehydrogenase or an expression vector comprising a nucleic acid encoding a DHB dehydrogenase.

In a specific aspect of the invention, the nucleic acid encoding the malate kinase is represented by SEQ ID NO: 13, SEQ ID NO:15, SEQ ID NO:17, SEQ ID NO:19, SEQ ID NO:21, SEQ ID NO:23, SEQ ID NO:25, SEQ ID NO:27, SEQ ID NO:38, SEQ ID NO:40, SEQ ID NO:42 or SEQ ID NO:44, the nucleic acid encoding the malate semialdehyde dehydrogenase is represented by SEQ ID 55, SEQ ID NO:57, SEQ ID NO:59, SEQ ID NO:61, SEQ ID NO:63, SEQ ID NO:65, or SEQ ID NO:67 and the nucleic acid encoding the DHB dehydrogenase is represented by SEQ ID NO: 73, SEQ ID NO:75, SEQ ID NO: 224, SEQ ID NO: 226 or SEQ ID NO: 82.

The term “host organism” is intended to mean any lower monocellular organism into which the chimeric gene(s), nucleic acid(s) or vector(s) according to the invention may be introduced in order to produce 2,4-DHB. Preferably, the host organism is a microorganism, in particular a fungus, for example of the Penicillium, Aspergillus and more particularly Aspergillus flavus, Chrysosporium or Trichoderma genus, a yeast, in particular of the Saccharomyces, Kluyveromyces or the Pichia genus and more particularly Zygosaccharomyces rouxii, a bacterium, for example of the Escherichia genus, in particular E. coli, or the Corynebacterium genus, more particularly Corynebacterium glutamicum, or of the Streptomyces genus or a baculovirus.

The host organism can be a host organism that naturally overproduces malate or succinate from sugars such as glucose or a host organism that was engineered to overproduce malate or succinate from sugars such as glucose and in which all potential membrane transporters that facilitate export of organic acids, such as malate, pyruvate, succinate, and fumarate have been deleted. The host organism can be an organism that was engineered to overproduce DHB and in which all membrane transporters that facilitate export of organic acids such as malate, pyruvate, succinate, and fumarate have been deleted. Examples of permeases that facilitate export of malate and other organic acids are Mae1 from Schizosaccharomyces pombe (Camarasa et al., 2001; Grobler et al., 1995), DctA from Bacillus subtilis (Groeneveld et al., 2010), Dct 1-4 from E. Coli, Jen1 from S. cerevisiae (Akita et al., 2000). For an expert it will be possible to identify candidate permeases in other microorganisms based on sequence homology. These constructions will serve to keep malate and other organic acids inside the cell to make them available for DHB production.

The expression “transformed host organism” is intended to mean a host organism which has incorporated into its genome, or in an extra chromosomal genetic element, for example a plasmid, at least one chimeric gene according to the invention, and consequently produces any one of malate kinase, malate semialdehyde dehydrogenase and/or DHB dehydrogenase in its tissues, or in a culture medium. To obtain the host organisms according to the invention, those skilled in the art may use one of the many known transformation methods.

One of these methods consists in bringing the cells of the host organisms to be transformed into contact with polyethylene glycol (PEG) and with the vectors according to the invention. Electroporation is another method, which consists in subjecting the cells to be transformed and the vectors of the invention to an electric field. Another method consists in directly injecting the vectors into the cells or the tissues by microinjection. The “biolistic” method may be used. It consists in bombarding cells or tissues with particles onto which the vectors of the invention are adsorbed (U.S. Pat. No. 4,945,050).

Several methods for transforming bacteria are described in the literature for Escherichia coli and other Gram-negative bacteria. Conjugation may also be used. For Gram-positive bacteria, electroporation may be used, and also protoplast transformation, in particular for bacteria of the Streptomyces genus.

Several methods for transforming fungi are also described in the literature. Protoplast transformation with PEG is described for Aspergillus in EP 0260762, and an adaptation of this method to the species Penicillium funiculosum is described in WO 00/36120. Transformation by restriction enzyme mediated integration, or REMI, is also known, as is protoplast transformation using bacteria of the Agrobacterium genus. Techniques for transforming yeasts are also described in the literature,

In a further aspect, the invention deals with a process of production of 2,4-DHB comprising the step of cultivating a transformed microorganism of the invention.

For the production of DHB various carbohydrates could be utilized individually or as a mixture such as glucose, fructose, sucrose, molasses, maltose, blackstrap molasses, starch hydrolysate (glucose, oligosaccharides), lactose, maltose, starch and starch hydrolysates, cellulose, cellulose hydrolysate, glycerol and certainly hydrocarbons, oils and fats such as soy bean oil, sunflower oil, groundnut oil and coconut oil as well as fatty acids such as e.g. palmitic acid, stearic acid and linoleic acid. Those substances may be used individually or as mixtures.

Various sources of nitrogen could be utilized individually or as mixtures for the commercial and pilot scale production, including inorganic compounds such as gaseous and aqueous ammonia, ammonium salts of inorganic or organic acids such as ammonium sulphate, ammonium nitrate, ammonium phosphate, ammonium chloride, ammonium acetate and ammonium carbonate. Alternatively, natural nitrogen containing organic materials like soybean-hydrolysate, soy protein HCl-hydrolysate (total nitrogen of about 7%), soy bean meal, soybean cake hydrolysate, corn steep liquor, casein hydrolysate, yeast extract, meat extract, malt extract, urea, peptones and amino acids may also be utilized

The production process can be carried out under aerobic, anaerobic, and oxygen limited conditions. It can be carried out as a fed-batch process or a batch process.

Said production of 2,4-DHB can be made by cultivating the host organism in media where malate (or another organic acid such as pyruvate, succinate, or fumarate) was added alone or together with another carbon source that ensures growth. Malate (and other organic acids) can be added either directly, or by designing a two-stage fermentation process where malate (or other organic acids) is produced in a first process stage by a malate-overproducing microorganism, and 2,4-DHB production is realised in the following stage by a host organism according to the invention.

Product separation and purification is very important factor enormously affecting overall process efficiency and product costs. Methods for product recovery commonly comprise the steps cell separation, as well as product purification, concentration and drying, respectively.

Cell Separation

Ultrafiltration and centrifugation can be used to separate cells from the fermentation medium. Cell separation from fermentation media is often complicated by high medium viscosity. Therefore, we can add additives such as mineral acid or alkali salts, or heating of the culture broth to optimize cell separation.

Product Recovery

A variety of ion-exchange chromatographic methods can be applied for the separation of DHB either before or after biomass removal. They include the use of primary cation exchange resins that facilitate separation of products according to their isoelectric point. Typically, the resin is charged with the solution, and retained product is eluted separately following increase of pH (eg by adding ammonium hydroxide) in the eluent. Another possibility represents the use of ion-exchange chromatography using fixed or simulated moving bed resins. Different chromatographic steps may have to be combined in order to attain adequate product purity. Those purification methods are more economical compared with a costly crystallization step, also providing additional advantages and flexibility regarding the form of final product.

Product Concentration and Drying

The purification process can also comprises a drying step which may involve any suitable drying means such as a spray granulator, spray dryer, drum dryer, rotary dryer, and tunnel dryer. Concentrated DHB solutions can be obtained by heating fermentation broths under reduced pressure by steam at 130° C. using a multipurpose concentrator or thin film evaporator.

Efficient production of DHB can be ensured by optimizing carbon flux repartitioning in the metabolic network of the host organism and by ensuring sufficient NADPH and ATP supply for the three enzymes of the DHB pathway. Channeling of carbon flux into a desired metabolic pathway and supply of NAD(P)H cofactor is commonly facilitated by deleting or alleviating competing natural fermentative pathways. Nonexclusive examples are

-   -   the optimization of malate production in S. cerevisiae by         impeding the formation of ethanol (by the deletion of pyruvate         decarboxylases (Zelle et al., 2008; Zelle et al., 2010).     -   the optimization of succinate or malate production in E. coli by         impeding the formation of lactate (e.g. deletion of IdhA), the         formation of acetate (e.g. deletion of pta, ackA), the formation         of ethanol (e.g. deletion of adhE), the formation of formate         (e.g. deletion of pflB, focA), the oxidation of pyruvate (e.g.         deletion of poxB), the degradation of malate (deletion of maeB         and scfA), the formation of succinate (e.g. deletion of frdBC),         the formation of methylglyoxal (deletion of mgsA) (Jantama et         al., 2008a; Jantama et al., 2008b; Lin et al., 2005; Sanchez et         al., 2005a; Zhang et al., 2011).

Another possibility to increase carbon flux and ATP supply for the production of organic acids is the engineering of the phosphoenolpyruvate (PEP)/pyruvate/oxaloacetate branch node (reviewed in (Sauer & Eikmanns, 2005)). Nonexclusive examples for metabolic engineering strategies that ensure the increase of carbon flux from phosphoenolpyruvate to oxaloacetate are

-   -   the optimization of malate production in S. cerevisiae by         impeding the function of pyruvate kinase and increasing the         activity of PEP carboxykinase (Zelle et al., 2010).     -   the optimization of succinate production in E. coli by         increasing the activity of natural or heterologously expressed         PEP carboxylase, PEP carboxykinase, or pyruvate carboxylase         (Millard et al., 1996; Sanchez et al., 2005b; Zhang et al.,         2009).

Another possibility to increase carbon flux and ATP supply for the production of organic acids in E. coli and other bacteria employing the PEP-consuming phosphotransferase system (PTS) for the initial phosphorylation step of glucose is the deletion of essential components of the PTS system (for example ptsl or ptsG) (Lin et al., 2005; Zhang et al., 2009). To ensure further glucose uptake in mutants carrying deleterious mutations of the PTS system, the activity of alternative glucose uptake systems (e.g. GalP) has to be ensured.

Another possibility to increase carbon flux into the desired pathways for the production of organic acids is the engineering of the citric acid and glyoxylate cycle. Non-exclusive examples are:

-   -   the optimization of succinic acid production in E. coli by         increasing the activity of isocitrate lyase (deletion of         transcriptional repressor iciR) (Lin et al., 2005; Sanchez et         al., 2005a).     -   the optimization of succinic acid production by the deletion of         isocitrate dehydrogenase, and/or succinate dehydrogenase (Lin et         al., 2005).

Another possibility to increase carbon flux into the desired pathways for the production of DHB is the expression of appropriate pyruvate dehydrogenases and citrate synthases in the production organism. Natural pyruvate dehydrogenase and citrate synthase of E. coli are inhibited by high intracellular NADH concentrations rendering these enzymes less active under anaerobic conditions. In E coli, the expression of a pyruvate dehydrogenase mutant that is insensitive to NADH resulted in the overproduction of acetyl-CoA under anaerobic conditions and modified carbon flux repartitioning between the fermentative end-products (acetate, lactate, ethanol, formate, and pyruvate) (Wang et al., 2010). The heterologous expression of the Bacillus subtilis citrate synthase which is insensitive to NADH increased succinic acid production in engineered E. coli strains (Sanchez et al., 2005a). In combination with the above described mutations, the use of the appropriate pyruvate dehydrogenases and citrate synthases (NADH sensitive or insensitive) enables the tuning of carbon flux repartitioning between glyoxylate and citric acid cycle reactions and fermentative pathways under anaerobic and aerobic conditions.

Another possibility to increase carbon flux through the DHB pathway is the deletion of enzymatic reactions that may degrade the pathway intermediates 4-phosphomalate, 4-malate semialdehyde. Candidate enzymes that may degrade malate semialdehyde are succinic semialdehyde dehydrogenases (sad, gabD), and other dehydrogenases that are able to oxidize C4 molecules with terminal aldehyde groups.

Another possibility to increase DHB productivity of the host organism is the deletion of metabolic reactions that degrade DHB. DHB is a competitive inhibitor of malic enzyme, thus, having comparatively high affinity for the active site of this enzyme (Rognstad & Katz, 1979). Therefore, DHB may be recognized by other enzymes and potentially degraded. These enzymes can be identified and deleted from the host organism.

When 2,4-DHB production is based on addition of malate or other organic acids, the 2,4-DHB-producing microorganisms should functionally express a membrane transport protein that facilitates uptake of malate (or other organic acids such as pyruvate, succinate, etc).

The following examples illustrate the invention. These examples are for purposes of illustration only and are not to be construed as limiting the scope of the invention in any manner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: (i) Reaction scheme that describes the conversion of (L)-malate into (L)-2,4-dihydroxybutyrate (DHB), and (ii) the analogy to the conversion of (L)-aspartate into (L)-homoserine.

FIGS. 2A-2C: Alignment of amino acid sequences of aspartate kinases from different organisms. (Ec_AKIII—aspartate kinase III (SEQ ID NO: 4), LysC, of E. coli, Ec_AKI (SEQ ID NO: 87)—aspartate kinase I, ThrA, of E. coli, Ec_AKII (SEQ ID NO: 88—aspartate kinase II, MetL, of E. coli, Mj—Methanococcus jannaschii (SEQ ID NO: 89), Tt—Thermus thermophilus (SEQ ID NO: 90), Cg—Corynebacterium glutamicum (SEQ ID NO: 91), At—Arabidopsis thaliana (SEQ ID NO: 92), Sc—Saccharomyces cerevisiae. (SEQ ID NO: 93)) The figure was produced using ClustalW2 (Larkin et al., 2007).

FIG. 3: Alignment of amino acid sequences of aspartate semialdehyde dehydrogenases from different organisms. (Ec—E. Coli (SEQ ID NO: 49), Mj—Methanococcus jannaschii (SEQ ID NO: 94), Tt—Thermus thermophilus (SEQ ID NO: 95), Bs—Bacillus subtilis (SEQ ID NO: 96), Cg—Corynebacterium glutamicum (SEQ ID NO: 97), At—Arabidopsis thaliana (SEQ ID NO: 98), Sc—Saccharomyces cerevisiae. (SEQ ID NO: 99)) The figure was produced using ClustalW2 (Larkin et al., 2007).

FIG. 4: GC chromatograms zooming on the region that corresponds to the retention time of DHB showing: (A) DHB standard (concentration=1 mM); (B) composition of Reaction A containing malate kinase (MK), malate semialdehyde dehydrogenase (MSA-Dh), and malate semialdehyde reductase (MSA-Red); (C) composition of control Reaction B (same as A but lacking MSA-Red); (D) composition of control Reaction C (same as A but lacking MSA-Dh).

FIG. 5: Relative activities of purified LysC E119G, LysC E119G E250K, LysC E119G T344M, LysC E119G S345L, LysC E119G T344M, and LysC E119G T352I mutants with respect to lysine concentration in the reaction buffer.

EXAMPLES Example 1 Test of Aspartate Kinases LysC and Hom3 from Escherichia coli and Saccharomyces cerevisiae, Respectively, for Aspartate and Malate Kinase Activity

Construction of plasmids containing wild-type genes of aspartate kinase: The plasmid pLYSCwt was constructed by amplifying the lysC gene by PCR using high fidelity polymerase Phusion™ (Finnzymes) and the direct and reverse primers ^(5′)CACGAGGTACATATGTCTGAAATTGTTGTCTCC^(3′) (SEQ ID NO: 1) and ^(5′)CTTCCAGGGGATCCAGT-ATTTACTCAAAC^(3′) (SEQ ID NO: 2) that introduce a NdeI and BamHI restriction sites upstream of the start codon and downstream of the stop codon, respectively. Genomic DNA from E. coli DH5a was used as the template. The PCR product was digested with NdeI and BamHI, ligated into the corresponding sites of the pET28a (Novagen) expression vector using T4 DNA ligase (Biolabs), and transformed into E. coli DH5α cells. The resulting pAKIIIwt plasmid was isolated and shown by DNA sequencing to contain the full-length lysC gene having the correct sequence (SEQ ID NO: 3). The corresponding protein is represented by SEQ ID NO: 4.

The plasmid pHOM3 wt was constructed by amplifying the HOM3 gene by PCR using high fidelity polymerase Phusion™ (Finnzymes) and the direct and reverse primers ^(5′)ATAATGCTAGCATGCCAATGGATTTCCAACC^(3′) (SEQ ID NO: 5) and ⁵TATAATGAATTCT-TAAATTCCAAGTCTTTTCAATTGTTC^(3′) (SEQ ID NO: 6) that introduce a NheI and an EcoRI restriction sites upstream of the start codon and downstream of the stop codon, respectively. Genomic DNA from S. cerevisiae BY4741 wt was used as the template. The PCR product was digested with NheI and EcoRI, and ligated into the corresponding sites of the pET28a (Novagen) expression vector using T4 DNA ligase (Biolabs), and transformed into E. coli DH5α cells. The resulting pHOM3 wt plasmid was isolated and shown by DNA sequencing to contain the full-length HOM3 gene having the correct sequence (SEQ ID NO: 7). The corresponding protein is represented by SEQ ID NO: 8.

Expression of enzymes: E. coli BL21 D3 star cells were transformed with the appropriate plasmids. Enzymes with an N-terminal hexa-His tag were expressed in 250 mL LB cultures that were inoculated from an overnight culture at OD₆₀₀ of 0.1 and grown to OD₆₀₀ of 0.6 before protein expression was induced by addition of 1 mM isopropyl β-D-1-thiogalactopyranoside (IPTG) to the culture medium. After 3 h of protein expression, cells were harvested by centrifugation at 13000 g for 10 min and stored at −20° C. until further analysis. Growth and protein expression were carried out at 37° C. Culture media contained 50 μg/L kanamycin.

Purification of enzymes: Frozen cell pellets of expression cultures were resuspended in 0.5 mL of breakage buffer (50 mM Hepes, 300 mM NaCl, pH 7.5) and broken open by four successive rounds of sonication (Bioblock Scientific, VibraCell™ 72437) with the power output set to 30%. Cell debris was removed by centrifuging the crude extracts for 15 min at 4° C. at 13000 g and retaining the clear supernatant. RNA and DNA were removed from the extracts by adding 15 mg/mL streptomycin (Sigma), centrifuging the samples at 13000 g for 10 min at 4° C. and retaining the supernatant. Clear protein extract was incubated for 1 h at 4° C. with 0.75 mL bed volumes of Talon™ Cobalt affinity resin (Clontech). The suspension was centrifuged at 700 g in a table top centrifuge and supernatant was removed. The resin was washed with 10 bed volumes of wash buffer (50 mM Hepes, 300 mM NaCl, 15 mM Imidazole, pH 7.5) before aspartate kinases were eluted with 0.5 mL of elution buffer (50 mM Hepes, 300 mM NaCl, 500 mM Imidazole, pH 7.5). Purity of eluted enzymes was verified by SDS-PAGE analysis.

Enzymatic assay: Aspartate or malate kinase activities were assayed by coupling ADP production in the kinase reactions to NADH oxidation in the presence of phosphoenolpyruvate, pyruvate kinase, and lactate dehydrogenase.

Reaction Scheme:

Aspartate (or Malate) Kinase aspartate (or malate)+ATP→4-phospho-(L)-aspartate (or 4-phospho-(L)-malate)+ADP

Pyruvate Kinase ADP+phosphoenolpyruvate→ATP+pyruvate

Lactate Dehydrogenase pyruvate+NADH→NAD⁺+lactate

The assay mixture contained 50 mM Hepes (pH 7.5), 50 mM KCl, 5 mM MgCl₂, 0.24 mM NADH, 0.96 mM ATP, 0.96 mM PEP, 9 μg/mL of lactate dehydrogenase (Sigma, L2500), 12.4 μg/mL pyruvate kinase (Sigma, P1506), and appropriate amounts of purified aspartate (malate) kinase. Reactions were started by adding 50 mM (L)-aspartate or (L)-malate. Enzymatic assays were carried out at 30° C. in 96-well flat bottomed microtiter plates in a final volume of 250 μL. The reactions were followed by the characteristic absorption of NADH at 340 nm in a microplate reader (BioRad 680XR).

Hydroxamate assay: To verify phosphorylation of the substrate, i.e. formation of an acylphosphate anhydride, by wild-type or mutated aspartate kinases, the product of the kinase reaction was incubated with hydroxylamine to form the corresponding aspartate or malate hydroxamate derivative. The assay mixture contained 120 mM Hepes (pH 8), 200 mM KCl, 10 mM ATP, 200 mM hydroxylamine, 10 mM aspartate or malate, and appropriate amount of purified protein. The reaction was stopped after 30 min by addition of an equal volume of 1.7% (w/v) FeCl₃ in 1 M hydrochloric acid. Formation of the hydroxamate-iron complex was verified by measuring its characteristic absorbance at 540 nm in a microtiter plate reader. Assay mixtures containing all components except for ATP were used as a blank.

Results: Purified LysC (without His-tag, SEQ ID NO: 4) and Hom3 (without His-tag, SEQ ID NO:7) enzymes exhibited aspartate kinase activity but were not able to phosphorylate malate as verified by the hydroxamate assay (Keng & Viola, 1996). Maximum activities for LysC and Hom3 on aspartate were 4.5 μmol/(min*mg_(prot)) and 1.6 μmol/(min*mg_(prot)), respectively. The Km value for aspartate was estimated with the method of Eadie and Hofstee by measuring initial reaction rates (v) at different substrate concentrations (c) and by extracting the slope of the v versus v/c plot. The Km of purified His-tagged LysC was estimated around 0.6 mM showing that the His-tagged protein has the same substrate affinity as the non-tagged purified enzyme which was reported to be 0.6 mM (Marco-Marin et al., 2003).

Example 2 Site Directed Mutagenesis of Aspartate Kinase LysC from Escherichia coli and Test of Mutant Enzymes for Malate Kinase Activity

Site-directed mutagenesis was carried out using the oligonucleotide pairs listed in Table 1 and the pLYSCwt (SEQ ID NO:3) plasmid as the template. Point mutations to change the amino acid sequence were introduced by PCR (Phusion 1 U, HF buffer 20% (v/v), dNTPs 2.5 mM, direct and reverse primers 1 μM each, template plasmid 200 ng, water) using the oligonucleotide pair listed in Table 1. Plasmids created by PCR contained a new restriction site for Nco1 (introduced using silent mutations) in addition to the functional mutation to facilitate identification of mutated clones. The PCR products were digested by DpnI at 37° C. for 1 h to remove template DNA, and transformed into NEB 5-alpha competent E. coli cells (NEB). The mutated plasmids were identified by restriction site analysis and verified to carry the desired mutations by DNA sequencing.

TABLE 1 Oligonucleotides used to mutate aspartate kinase lysC from E. coli in position E119. Mutation Sequence 5′-3′ E119nnk GCTGGTCAGCCATGGCNNNCTGATGTCGACCCTGC (SEQ ID NO: 10) GCAGGGTCGACATCAGNNNGCCATGGCTGACCAGC (SEQ ID NO: 11)

The sequence representing a mutation in position 119 can be represented by SEQ ID NO:9, wherein the residue in position 119 is X, X being anyone of the 19 naturally occurring amino acid (except glutamine).

Mutant enzymes were expressed, purified and tested for aspartate and malate kinase activity as described in Example 1. Results are summarized in Table 2.

TABLE 2 Characterization of mutant enzymes for malate kinase activities. Values correspond to the average from at least two independent experiments. Amino acid in position 119 Vmax Km (Corresponding SEQ ID NO:) [μmol/(mg*min]] [mM]] C (SEQ ID NO:12) 0.97 19.7 G (SEQ ID NO:14) 0.49 16.0 N (SEQ ID NO:16) 0.13 27.1 P (SEQ ID NO: 18) 0.71 19.0 Q (SEQ ID NO:20) 0.01 39.9 S (SEQ ID NO:22) 0.83 15.7 T (SEQ ID NO:24) 0.33 26.8 V (SEQ ID NO:26) 0.29 39.7

None of the mutants listed in Table 2 had activity on aspartate.

The results show that aspartate kinase can be transformed into malate kinase by replacing the conserved glutamate at position 119 by cysteine, glycine, asparagine, proline, glutamine, serine, threonine, or valine.

The corresponding nucleic acid sequences of the enzyme listed in Table 2 are SEQ ID NO:13, SEQ ID NO:15, SEQ ID NO:17, SEQ ID NO:19, SEQ ID NO:21, SEQ ID NO:23, SEQ ID NO:25 and SEQ ID NO:27.

Example 3 Construction of a Malate Kinase with Strongly Decreased Sensitivity for Inhibition by Lysine

Site-directed mutagenesis was carried out using the oligonucleotide pairs listed in Table 3 and the pLYSC_E119G plasmid as the template (The pLYSC_E119G plasmid was obtained as described in Example 2 by introducing the following changes in the DNA sequence of the lysC gene: (SEQ ID NO: 15). Point mutations to change the amino acid sequences were introduced by PCR (Phusion 1U, HF buffer 20% (v/v), dNTPs 2.5 mM, direct and reverse primers 1 μM each, template plasmid 200 ng, water) using the oligonucleotide pairs listed in Table 1. When possible, plasmids created by PCR contained new restriction sites (introduced using silent mutations) in addition to the functional mutation to facilitate identification of mutated clones. The PCR products were digested by DpnI at 37° C. for 1 h to remove template DNA, and transformed into NEB 5-alpha competent E. coli cells (NEB). The mutated plasmids were identified by restriction site analysis and verified to carry the desired mutations by DNA sequencing.

TABLE 3 Oligonucleotides used to mutate malate kinase lysC E119G from E. coli. Mutation Sequence 5′-3′ E250K GCGTTTGCCGAAGCGGCAAAGATGGCCACTTTTG  (SEQ ID NO: 28) CAAAAGTGGCCATCTTTGCCGCTTCGGCAAACGC  (SEQ ID NO: 29) T344M GGTAGATCTAATCACCATGTCAGAAGTGAGCGTGG  (SEQ ID NO: 30) CCACGCTCACTTCTGACATGGTGATTAGATCTACC  (SEQ ID NO: 31) 5345L GGTAGATCTAATCACCACGTTAGAAGTGAGCGTGGC (SEQ ID NO: 32) GCCACGCTCACTTCTAACGTGGTGATTAGATCTACC (SEQ ID NO: 33) T344M GGTAGATCTAATCACCATGTCAGAAGTGAGCGTGG  (SEQ ID NO: 34) CCACGCTCACTTCTGACATGGTGATTAGATCTACC  (SEQ ID NO: 35) T352I GTCAGAAGTGAGCGTGGCATTAATTCTAGATACCAC (SEQ ID NO: 36) GTGGTATCTAGAATTAATGCCACGCTCACTTCTGAC (SEQ ID NO: 37)

The nucleic acid sequence of the protein LysC E119G comprising an additional mutation corresponding to (i) the replacement of the glutamic acid in position 250 by a lysine is represented by SEQ ID NO: 38; its corresponding amino acid sequence is represented by SEQ ID NO: 39; (ii) the replacement of the threonine in position 344 by methionine is represented by SEQ ID NO: 40; its corresponding amino acid sequence is represented by SEQ ID NO: 41; (iii) the replacement of the threonine in position 352 by isoleucine is represented by SEQ ID NO: 42; its corresponding amino acid sequence is represented by SEQ ID NO: 43, (iv) the replacement of the serine in position 345 by leucine is represented by SEQ ID NO: 44; its corresponding amino acid sequence is represented by SEQ ID NO: 45.

Expression and purification of enzymes: Protein expression for the His-tagged enzymes LysC E119G, LysC E119G E250K, LysC E119G T344M, LysC E119G S345L, LysC E119G T352I was carried out as described in Example 1.

Enzymatic assay: Malate kinase activities were assayed as described in Example 1. Lysine concentration in the reaction buffer was varied.

Results: The introduction of mutations E250K, T344M or S345L into LysC E119G renders the malate kinase activity largely insensitive to elevated lysine concentrations (See FIG. 4).

Example 4 Test of Aspartate Semialdehyde Dehydrogenases Asd from Escherichia coli for Aspartate and Malate Semialdehyde Dehydrogenase Activity

Construction of Plasmids Containing Wild-Type Genes of Aspartate Semialdehyde Dehydrogenase: The plasmid pASDwt was constructed by amplifying the asd gene of E. coli by PCR using high fidelity polymerase Phusion™ (Finnzymes) and the direct and reverse primers ^(5′)ATAATGCTAGCATGAAAAATGTTGGTTTTATCGG^(3′) (SEQ ID NO: 46) and ^(5′)ATAATGGATCCTTACGCCAGTTGACGAAGC^(3′) (SEQ ID NO: 47) that introduce a NheI and BamHI restriction site upstream of the start codon and downstream of the stop codon, respectively. Genomic DNA from E. coli DH5α was used as the template. The PCR product was digested with NheI and BamHI, ligated into the corresponding sites of the pET28a (Novagen) expression vector using T4 DNA ligase (Biolabs), and transformed into E. coli DH5α cells. The resulting pASDwt plasmid was isolated and shown by DNA sequencing to contain the full-length asd gene having the correct sequence (SEQ ID NO: 48). The corresponding amino acid sequence of said enzyme is represented by SEQ ID NO: 49.

Expression and purification of enzymes: Protein expression for the His-tagged enzymes Asd was carried out as described in Example 1.

Enzymatic assay: Aspartate or malate semialdehyde dehydrogenase activities were assayed in the reverse biosynthetic direction by following the reduction of NADP during the oxidation of aspartate or malate semialdehyde to 4-phospho-(L)-aspartate or 4-phospho-(L)-malate, respectively (Roberts et al., 2003). (L)-aspartate semialdehyde (or (L)-malate semialdehyde)+NADP+Pi→4-phospho-(L)-aspartate (or 4-phospho-(L)-malate)+NADPH

The assay mixture contained 200 mM Hepes (pH 9), 50 mM K₂HPO₄, 0.25 mM NADP. Reactions were started by adding (L)-aspartate semialdehyde or (L)-malate semialdehyde. (L)-Aspartate semialdehyde was added in the form of L-aspartic acid β-semialdehyde hydrate trifluoroacetate (maintained at pH3 to prevent degradation) which is a suitable substrate for enzymatic tests of homoserine dehydrogenase and aspartate semialdehyde dehysrogenase (Roberts et al., 2003). Unstable malate semialdehyde was produced freshly prior to the enzymatic tests by the deprotection of the stable malate semialdehyde derivative 2-[(4S)-2,2-dimethyl-5-oxo-1,3-dioxolan-4-yl]acetaldehyde (DMODA). Malate semialdehyde was obtained by incubating DMODA in 2M hydrochloric acid for 15 min at 25° C., and evaporation of the released acetone (35° C., 50 mbar). The pH of the malate semialdehyde solution was fixed at 3 using sodium bicarbonate.

Enzymatic assays were carried out in 96-well flat bottomed microtiter plates in a final volume of 250 μL at 30° C. The reactions were followed by the characteristic absorption of NADPH at 340 nm in a microplate reader (BioRad 680XR).

Results: His-tagged wild-type aspartate semialdehyde dehydrogenase, Asd, oxidised (L)-aspartate semialdehyde to 4-phospho-(L)-aspartate with a maximum specific activity of 160 μmol/(min*mg_(prot)). On (L)-malate semialdehyde the enzyme had an activity of 0.01 μmol/(min*mg_(prot)).

Example 5 Site Directed Mutagenesis of Aspartate Semialdehyde Dehydrogenase Asd from Escherichia coli and Test of Mutant Enzymes for Malate Semialdehyde Dehydrogenase Activity

Point mutations in the amino acid sequence of Asd were introduced using the pASDwt plasmid as the template and following the protocol outlined in Example 2. The oligonucleotide pairs listed in Table 4 were used to mutate the glutamate residue in position 241 or the threonine residue in position 136. The mutated plasmids were identified by restriction site analysis and verified to carry the desired mutations by DNA sequencing.

The Asd protein mutated in position 241 can be represented by SEQ ID NO: 68 wherein the residue in position 241 is X, X being anyone of the other 19 biologically occurring amino acid (except glutamine).

TABLE 4 Oligonucleotides used to mutate aspartate semialdehyde dehydrogenase Asd from E. coli in position E241 and T136. Mutation Sequence 5′-3′ E241nnn AGCTCGATAACGGTCAGAGTCGANNNGAGTGG AAAGGGCAGGCGG (SEQ ID NO: 50) CCGCCTGCCCTTTCCACTCNNNTCGACTCTGA CCGTTATCGAGCT (SEQ ID NO: 51) T136N TTTTGTTGGCGGTAACTGTAACGTGTCCCTGA TGTTG (SEQ ID NO: 52) CAACATCAGGGACACGTTACAGTTACCGCCAA CAAAA (SEQ ID NO: 53)

Results: Activities and Km values of Asd mutated in position E241 are summarized in Table 5. Asd mutants where glutamate 241 was replaced by alanine, cysteine, glycine, histidine, isoleucine, methionine, or glutamine oxidised (L)-aspartate-4-semialdehyde to 4-phospho-(L)-aspartate with a significantly higher maximum specific activity than the wild-type enzyme. The double mutant Asd E241Q T136N (SEQ ID NO:231) had a maximum specific activity of 0.25 μmol/(min*mg_(prot)) and a Km of 0.25 mM.

TABLE 5 Characterization of mutant enzymes for malate semialdehyde dehydrogenase activities. Values correspond to the average from at least two independent experiments. Amino acid in position 241 Vmax Km* (Corresponding SEQ ID NO:) [μmol/(mg*min]] [mM] A (SEQ ID NO:54) 0.09 0.378 C (SEQ ID NO:56) 0.18 0.5 E (= wt) (SEQ ID NO:49) 0.01 G (SEQ ID NO:58) 0.09 0.18 H (SEQ ID NO:60) 0.10 0.8 I (SEQ ID NO:62) 0.10 0.23 M (SEQ ID NO:64) 0.15 0.43 Q (SEQ ID NO:66) 0.39 0.52 *Km values were only estimated for selected mutants

The corresponding nucleic acids are represented by SEQ ID NO:55, SEQ ID NO:57, SEQ ID NO:48, SEQ ID NO:59, SEQ ID NO:61, SEQ ID NO:63, SEQ ID NO:65 and SEQ ID NO:67.

The double mutant Asd E241Q T136N has a nucleic acid sequence represented by SEQ ID NO: 230.

Example 6 Identification of a 2,4 DHB Dehydrogenase

To identify a suitable 2,4 DHB dehydrogenase, beta-hydroxyacid dehydrogenases from different biological sources were tested for their ability to reduce malate semialdehyde. Among the tested enzymes were the methylbutyraldehyde reductase, Ypr1 (Ford & Ellis, 2002))(SEQ ID NO: 73 and SEQ ID NO: 74), from Saccharomyces cerevisiae; and the succinic semialdehyde reductase, Ms-Ssr from Metallosphaera sedula (Kockelkorn & Fuchs, 2009)(SEQ ID NO: 75 and SEQ ID NO: 76). The genes YPR1 and Ms-SSR were amplified using primers listed in Table 6 and cloned into vector pET28 (restriction enzymes see Table 3) yielding plasmids pYPR1 and pMs-SSR, respectively. The proteins were expressed and purified as described in Example 1.

TABLE 6 Primers and restriction enzymes  used to clone candidate beta-hydroxyacid dehydrogenases Restriction Enzyme Accession No Primer 5′-3′ enzymes Ms-SSR GI:146304190 TATAATGCTAGCATG NheI AAAGCTGCAGTACTT CA (SEQ ID NO: 69) TATAATGAATTCTTA EcoRI CGGGATTATGAGACT TC (SEQ ID NO: 70) YPR1 GI:6320576 TATAATGCTAGCATG NheI CCTGCTACGTTAAAG AA (SEQ ID NO: 71) TATAATGAGCTCTCA SacI TTGGAAAATTGGGAA GG (SEQ ID NO: 72) Test for Malate Semialdehyde Reductase Activity: Reaction: (L)-Malate semialdehyde+NAD(P)H→(L)-2,4-dihydroxybutyric acid+NAD(P)

The assay mixture contained 200 mM Hepes (pH 7.5), 50 mM KCl, 5 mM MgCl₂, 0.24 mM NADH or NADPH, and appropriate amounts of purified enzyme. Reactions were started by adding 10 mM (L)-malate semialdehyde (malate semialdehyde was prepared freshly for each test, see Example 4). Enzymatic assays were carried out at 30° C. in 96-well flat bottomed microtiter plates in a final volume of 250 μL. The reactions were followed by the characteristic absorption of NAD(P)H at 340 nm in a microplate reader (BioRad 680XR). Results are listed in Table 7.

TABLE 7 Reducing activity of selected beta-hydroxyacid dehydrogenases on malate semialdehyde (Results represent the average of at least two independent experiments). Activity Activity on malate on malate semialdehyde semialdehyde (cofactor (cofactor NADH) NADPH) Reported [μmol/ [μmol/ Enzyme Origin function (min*mg_prot)] (min*mg_prot)] Ms-SSR M. sedula Succinic 4.9 4.9 (SEQ ID semialdehyde NO: 76) reductase YPR1 S. cerevisiae Methyl- nd 0.19 (SEQ ID butyraldehyde NO: 74) reductase

The succinic semialdehyde dehydrogenase from M. sedula and the methylbutyraldehyde reductase from S. cerevisiae have malate semialdehyde reductase activity. The Km of Ms-SSR for malate semialdehyde was 1.1 mM.

Example 7 Site Directed Mutagenesis of Succinic Semialdehyde Reductase from M. sedula

Site-directed mutagenesis was carried out using the oligonucleotide pairs listed in Table 8 and the pMs-SSR plasmid as the template. Point mutations to change the amino acid sequences were introduced by PCR (Phusion 1U, HF buffer 20% (v/v), dNTPs 2.5 mM, direct and reverse primers 1 μM each, template plasmid 200 ng, water). When possible, plasmids created by PCR contained new restriction sites (introduced using silent mutations) in addition to the functional mutation to facilitate identification of mutated clones. The PCR products were digested by DpnI at 37° C. for 1 h to remove template DNA, and transformed into NEB 5-alpha competent E. coli cells (NEB). The mutated plasmids were identified by restriction site analysis and verified to carry the desired mutations by DNA sequencing. Table 9 summarizes kinetic parameters of the mutants. The results demonstrate that the double mutant Ms-SSR H39R N43H (SEQ ID NO: 81, SEQ ID NO: 82) has improved affinity for malate semialdehyde when compared to the wild type enzyme.

TABLE 8 Primer pairs used to mutate M. sedula succinic semialdehyde reductase (Ms-SSR) Restriction Mutation Primer 5′-3′ enzymes H39R gtcaaggcaaccggtctctg NheI tcgctccgacgtcaatg (SEQ ID NO: 77) cattgacgtcggagcgacag agaccggttgccttgac (SEQ ID NO: 78) N43H ggctctgtcactccgacgta NheI catgtctttgaggggaaaac (SEQ ID NO: 79) gttttcccctcaaagacatg tacgtcggagtgacagagcc (SEQ ID NO: 80)

TABLE 9 Summary of kinetic parameters of M. sedula succinic semialdehyde reductase (Ms-SSR) mutants (Results represent the average of at least two independent experiments). Maximum activity Km Mutant [μmol/(min* mg_(prot))] [mmol/L] Wild type ((SEQ ID NO: 76) 4.9 1.1 H39R (SEQ ID NO:225) 1.7 0.5 N43H (SEQ ID NO:227) 4.3 2.5 H39R N43H (SEQ ID NO: 81) 4.7 0.4

The corresponding nucleic sequences are represented by SEQ ID NO: 224, SEQ ID NO: 226 and SEQ ID NO: 82.

Example 8 In vitro Production of DHB

The enzymes malate kinase (LysC E119G, SEQ ID NO: 15), malate semialdehyde dehydrogenase (Asd E241Q; SEQ ID NO: 67), and malate semialdehyde reductase (Ms SSrR, SEQ ID NO: 76) were expressed and purified as described in Example 1. Production of DHB was demonstrated in vitro by adding 50 mM malate to a reaction mixture that contained 50 mM Hepes (pH 7.5), 50 mM KCl, 5 mM MgCl₂, 1 mM NADPH, 180 μg/mL of malate kinase (Lys E119G), 325 μg/mL of malate semialdehyde dehydrogenase (Asd E241Q), and 130 μg/mL of malate semialdehyde reductase (Ms_Ssr) (Reaction A). Control reactions contained all components but were lacking either malate semialdehyde reductase (Reaction B) or malate semialdehyde dehydrogenase (Reaction C). After 30 min of incubation at 30° C., the reaction mixture was analysed by gas chromatography [CPG Varian Series 430; equipped with FID detector; autosampler CP8400; splitless injector 1177 (230° C.); column: CP-WAX58/FFAP, 30 m×0.53 mm, d_(f) 0.50 μm; and liner: Inlet Sleeve, gooseneck 6.5×78.5×4 mm GWOL (Varian). Carrier gas was nitrogen at a flow rate of 25 mL/min. Flame ionization was carried out using an air-hydrogen mixture (flow rates were 300 mL/min and 30 mL/min, respectively). Detector temperature was 240° C. Injected sample volume was 1 μL. Temperature program is provided in Table 10.

TABLE 10 Temperature program for analysis of reaction mixtures Column temperature Hold Gradient Runtime [° C.] [min] [° C./min] [min] 90 0 0 0 115 1.8 30 2.63 160 2 2 27.13 230 1 50 29.53

DHB production was detected in reaction A (presence of all enzymes), but was absent in control reaction B and C (FIG. 5).

Example 9 Optimization of the Coding Sequence of M. sedula Succinic Semialdehyde Reductase for its Expression in E. coli

The coding sequence of M. sedula succinic semialdehyde reductase including the mutations H39R and N43H was optimized for maximum expression in E. coli, using the GeneOptimizer® software. The synthetic gene was produced by GeneArt® Gene Synthesis (Invitrogen Life Technologie). NheI and EcoRI restriction sites were introduced upstream of the start codon and downstream of the stop codon, respectively, allowing direct cloning into pET28a+ (Novagen).

The resulting pSSR-H39RN43H-opt plasmid was isolated and shown by DNA sequencing to contain the full-length M sedula SSR H39R N43H gene having the correct sequence (SEQ ID NO:228).

Example 10 Construction of a Plasmid that Facilitates the Simultaneous Expression of Malate Kinase (Mutant of the lysC Gene from E. coli), Malate Semialdehyde Dehydrogenase, (Mutant of the Asd Gene from E. coli), and DHB Dehydrogenase (Mutant of the M. sedula Succinic Semialdehyde Reductase Gene) Using E. coli as the Host Organism

The plasmid pLYSC-E119G E250K (SEQ ID NO:38) was used as the backbone for the operon construction. A DNA fragment containing the pET28 (Novagen) ribosome binding site (rbs) and the coding region of ASD-E241Q was obtained by PCR (high fidelity polymerase Phusion™ (Finnzymes)) using pASD-E241Q (SEQ ID NO: 55 as the template, and the direct and reverse primers ^(5′)ATAAGGATCCGTTTAACTTTAAGAAGGAGATATACCATGGG^(3′) (SEQ ID NO: 83) and ^(5′)ATAAGAATTCTTACGCCAGTTGACGAAG^(3′) (SEQ ID NO: 84) that introduced a BamHI and a EcoRI restriction site upstream of the rbs and downstream of the stop codon, respectively. The PCR products were digested with BamHI and EcoRI, ligated into the corresponding sites of pLYSC-E119G E250K, using T4 DNA ligase (Biolabs), and transformed into E. coli DH5α cells. The resulting pLYSC-E119G-E250K_ASD-E241Q plasmid was isolated and shown by DNA sequencing to have the correct sequence.

A DNA fragment containing the pET28 ribosome binding site (rbs) and the coding region of the codon-optimized Ms-SSR-H39RN43H-opt was obtained by PCR using pSSR-H39RN43H-opt as the template, and the direct and reverse primers ^(5′)ATAAGCGGCCGCGTTTAACTTTAAGAAGGAGATAT^(3′) (SEQ ID NO:85) and ^(5′)TATAAACTCGAGCTTACGGAATAATCAGG^(3′) (SEQ ID NO: 86) that introduced a NotI and a PspXI restriction site upstream of the rbs and downstream of the stop codon, respectively. The PCR products were digested with NotI and PspXI, ligated into the corresponding sites of pLYSC-E119G-E250K_ASD-E241Q, using T4 DNA ligase (Biolabs), and transformed into E. coli DH5α cells. The resulting pET28-DHB plasmid (SEQ ID NO: 229) was isolated and shown by DNA sequencing to have the correct sequence.

The 5′ upstream promoter region simultaneously regulating the expression of the three genes (ie T7 promoter in pET28-DHB) can be replaced with any other promoter, inducible or constitutive, by digesting pET28-DHB with SphI and XbaI and cloning another promoter region with suitable restriction sites. As an example for the use of an inducible promoter, the T7 promoter of the pET28-DHB backbone was replaced by the tac promoter whose characteristics allow for protein expression in the presence of glucose (de Boer et al., 1983). The tac promoter was obtained from plasmid pEXT20 (Dykxhoorn et al., 1996) by digesting the plasmid with SphI and XbaI. The DNA fragment containing the promoter was purified and cloned into the SphI/XbaI digested pET28-DHB plasmid. The resulting pTAC-DHB plasmid was isolated and shown by DNA sequencing to have the correct sequence.

TABLE 11 List of plasmid constructed in this study Plasmid Regulation Features pET28-DHB T7 lysC-E119G-E250K, asd-E241Q, Ms_SSR- H39R-N43H codon optimized pTAC-DHB tac lysC-E119G-E250K, asd-E241Q, Ms_SSR- H39R-N43H codon optimized

Example 11 Construction of E. coli Strains to Optimise Carbon Flux Repartitioning and NADPH-cofactor Supply for Fermentative DHB Production

Several genes were disrupted in E. coli strain MG1655 in order to optimise carbon flux repartitioning and cofactor supply for DHB production. Gene deletions were carried out using the lambda red recombinase method according to Datsenko et al. (Datsenko & Wanner, 2000).

The deletion cassettes were prepared by PCR using high fidelity polymerase Phusion™ (Finnzymes), and the FRT-flanked kanamycin resistance gene (kan) of plasmid pKD4 as the template (Datsenko & Wanner, 2000). Sense primers contained sequences corresponding to the 5′ end of each targeted gene (underlined) followed by 20 by corresponding to the FRT-kan-FRT cassette of pKD4. Anti-sense primers contained sequences corresponding to the 3′ end region of each targeted gene (underlined) followed by 20 bp corresponding to the cassette. The primers are described in Table 12. PCR products were digested with DpnI and purified prior to transformation.

E. coli MG1655 strain was rendered electro-competent by growing the cells to an OD₆₀₀ of 0.6 in LB liquid medium at 37° C., concentrating the cells 100-fold and washing twice with ice-cold 10% glycerol. The cells were transformed with plasmid pKD46 (Datsenko & Wanner, 2000) by electroporation (2.5 kV, 200 Ω, 25 μF, in 2 mm gap cuvettes). Transformants were selected at 30° C. on ampicillin (100 μg/mL) LB solid medium.

Disruption cassettes were transformed into electro-competent E. coli strains harbouring the lambda Red recombinase-expressing plasmid pKD46. The cells were grown at 30° C. in liquid SOB medium containing ampicillin (100 μg/mL). The lambda red recombinase system was induced by adding 10 mM arabinose when OD₆₀₀ of the cultures reached 0.1. Cells were further grown to an OD₆₀₀ of 0.6 before they were harvested by centrifugation, washed twice with ice-cold 10% glycerol, and transformed with the disruption cassette by electroporation. After an overnight phenotypic expression at 30° C. in LB liquid medium, cells were plated on solid LB medium containing 25 μg/mL kanamycin. Transformants were selected after cultivation at 30° C.

The gene replacement was verified by colony PCR using Crimson Taq polymerase (NEB). A first reaction was carried out with the flanking locus-specific primers (see tables 13) to verify simultaneous loss of the parental fragment and gain of the new mutant specific fragment. Two additional reactions were done by using nearby locus-specific primers with the respective common test primer k1 rev, or k2 for (see Table 13) within the FRT-kanamycin resistance cassette (sense locus primer/k1 rev and k2 for/reverse locus primer).

The resistance gene (FRT-kan-FRT) was subsequently excised from the chromosome using the FLP recombinase-harbouring plasmid pCP20 (Cherepanov & Wackernagel, 1995) leaving a scar region containing one FRT site. pCP20 is an ampicillin and CmR plasmid that shows temperature-sensitive replication and thermal induction of FLP recombinase synthesis. Kanamycin resistant mutants were transformed with pCP20, and ampicillin-resistant transformants were selected at 30° C. Transformants were then grown on solid LB medium at 37° C. and tested for loss of all antibiotic resistances. Excision of the FRT-kanamycin cassette was analysed by colony PCR using crimson taq polymerase and the flanking locus-specific primers (Table 13). Multiple deletions were obtained by repeating the above described steps.

Strains carrying single or multiple deletions were rendered electro-competent as described above, transformed with the pTAC-DHB plasmid which allows for the IPTG-inducible expression of the DHB pathway enzymes (see Example 10), and selected on solid LB medium containing 50 μg/mL kanamycin.

The plasmid pACT3-pck harbouring the PEP carboxykinase encoding pck gene of E. coli was constructed by amplifying the pck coding sequence using genomic DNA from E. coli MG1655 as the template and the forward and reverse primers, respectively, ^(5′)ATAATCCCGGGATGCGCGTTAACAATGGTTTGACC^(3′) (SEQ ID NO: 100) and ^(5′)ATAATTCTAGATTACAGTTTCGGACCAGCCG^(3′) (SEQ ID NO: 101). The DNA fragment was digested with Xmal and XbaI, ligated into the corresponding sites of the pACT3 expression vector (Dykxhoorn et al., 1996) using T4 DNA ligase (Biolabs), and transformed into E. coli DH5α cells. The transformants were selected on solid LB medium containing chloramphenicol (25 μg/mL). The resulting plasmid was isolated and correct insertion of the pck gene was verified by sequencing. Plasmids pACT3-aceA, pACT3-ppc, pACT3-galP, pACT3-pykA and pACT3-pyc harbouring, respectively, aceA, ppc, galP, or pykA (all E. coli) or pycA from Lactococcus lactis were constructed analogously using the primers listed in Table 14.

The above mentioned pACT3-derived plasmids and the pTAC-DHB plasmid were transformed into E. coli MG1655 mutants carrying combinations of the deletions listed in Table 12. Transformants containing both plasmids were selected on solid LB medium containing chloramphenicol (25 μg/mL) and kanamycin (50 μg/mL). Examples for constructed strains are listed in Table 15.

TABLE 12 Primers used for gene disruptions. Sequences homologous to target genes are underlined. Gene Primer Sequence ldhA Δ_ldhA_for Gaaggttgcgcctaca ctaagcatagttgttg atgagtgtaggctgga gctgcttc (SEQ ID NO: 102) Δ_ldhA_rev Ttaaaccagttcgttc gggcaggtttcgcctt tttcatgggaattagc catggtcc (SEQ ID NO: 103) adhE Δ_adhE_for Atggctgttactaatg tcgctgaacttaacgc actcgtagagcgtgtg taggctggagctgctt c (SEQ ID NO: 104) Δ_adhE_rev Ttaagcggattttttc gcttttttctcagctt tagccggagcagccat atgaatatcctcctta g (SEQ ID NO: 105) ackA Δ_ackA_for atgtcgagtaagttag tactggttctgaactg cggtagttcttcagtg taggctggagctgctt c (SEQ ID NO: 106) Δ_ackA_rev tcaagcagtcaggcgg ctcgcatcttgcgcga taaccagttcttccat atgaatatcctcctta g (SEQ ID NO: 107) focA- Δ_focA- ttactccgtatttgca pflB pflB_for taaaaaccatgcgagt tacgggcctataagtg taggctggagctgctt c (SEQ ID NO: 108) Δ_focA- atagattgagtgaagg pflB_rev tacgagtaataacgtc ctgctgctgttctcat atgaatatcctcctta g (SEQ ID NO: 109) pta Δ_pta_for gtgtcccgtattatta tgctgatccctaccgg aaccagcgtcggtgtg taggctggagctgctt c (SEQ ID NO: 110) Δ_pta_rev ttactgctgctgtgca gactgaatcgcagtca gcgcgatggtgtacat atgaatatcctcctta g (SEQ ID NO: 111) poxB Δ_poxB_for atgaaacaaacggttg cagcttatatcgccaa aacactcgaatcggtg taggctggagctgctt c (SEQ ID NO: 112) Δ_poxB_rev ttaccttagccagttt gttttcgccagttcga tcacttcatcacccat atgaatatcctcctta g (SEQ ID NO: 113) sad Δ_sad_for atgaccattactccgg caactcatgcaatttc gataaatcctgccgtg taggctggagctgctt c (SEQ ID NO: 114) Δ_sad_rev tcagatccggtctttc cacaccgtctggatat tacagaattcgtgcat atgaatatcctcctta g (SEQ ID NO: 115) gabD Δ_gabD_for atgaaacttaacgaca gtaacttattccgcca gcaggcgttgattgtg taggctggagctgctt c (SEQ ID NO: 116) Δ_gabD_rev ttaaagaccgatgcac atatatttgatttcta agtaatcttcgatcat atgaatatcctcctta g (SEQ ID NO: 117) gadA Δ_gadA_for atggaccagaagctgt taacggatttccgctc agaactactcgatgtg taggctggagctgctt c (SEQ ID NO: 118) Δ_gadA_rev tcaggtgtgtttaaag ctgttctgctgggcaa taccctgcagtttcat atgaatatcctcctta g (SEQ ID NO: 119) gadB Δ_gadB_for atggataagaagcaag taacggatttaaggtc ggaactactcgatgtg taggctggagctgctt c (SEQ ID NO: 120) Δ_gadB_rev tcaggtatgtttaaag ctgttctgttgggcaa taccctgcagtttcat atgaatatcctcctta g (SEQ ID NO: 121) gadC Δ_gadC_for atggctacatcagtac agacaggtaaagctaa gcagctcacattagtg taggctggagctgctt c (SEQ ID NO: 122) Δ_gadC_rev ttagtgtttcttgtca ttcatcacaatatagt gtggtgaacgtgccat atgaatatcctcctta g (SEQ ID NO: 123) sfcA Δ_sfcA_for atggaaccaaaaacaa aaaaacagcgttcgct ttatatcccttacgtg taggctggagctgctt c (SEQ ID NO: 124) Δ_sfcA_rev ttagatggaggtacgg cggtagtcgcggtatt cggcttgccagaacat atgaatatcctcctta g (SEQ ID NO: 125) maeB Δ_maeB_for atggatgaccagttaa aacaaagtgcacttga tttccataaatttgta taggctggagctgctt c (SEQ ID NO: 126) Δ_maeB_rev ttacagcggttgggtt tgcgcttctaccacgg ccagcgccaccatcat atgaatatcctcctta g (SEQ ID NO: 127) ppc Δ_ppc_for atgaacgaacaatatt ccgcattgcgtagtaa tgtcagtatgctcgtg taggctggagctgctt c (SEQ ID NO: 128) Δ_ppc_rev ttagccggtattacgc atacctgccgcaatcc cggcaatagtgaccat atgaatatcctcctta g (SEQ ID NO: 129) pykA Δ_pykA_for atgtccagaaggcttc gcagaacaaaaatcgt taccacgttaggcgtg taggctggagctgctt c (SEQ ID NO: 130) Δ_pykA_rev ttactctaccgttaaa atacgcgtggtattag tagaacccacggtcat atgaatatcctcctta g (SEQ ID NO: 131) pykF Δ_pykF_for atgaaaaagaccaaaa ttgtttgcaccatcgg accgaaaaccgaagtg taggctggagctgctt c (SEQ ID NO: 132) Δ_pykF_rev ttacaggacgtgaaca gatgcggtgttagtag tgccgctcggtaccat atgaatatcctcctta g (SEQ ID NO: 133) mgsA Δ_mgsA_for atggaactgacgactc gcactttacctgcgcg gaaacatattgcggtg taggctggagctgctt c (SEQ ID NO: 134) Δ_mgsA_rev ttacttcagacggtcc gcgagataacgctgat aatcggggatcagcat atgaatatcctcctta g (SEQ ID NO: 135) iclR Δ_iclR_for atggtcgcacccattc ccgcgaaacgcggcag aaaacccgccgttgtg taggctggagctactt c (SEQ ID NO: 136) Δ_iclR_rev tcagcgcattccaccg tacgccagcgtcactt ccttcgccgctttcat atgaatatcctcctta g (SEQ ID NO: 137) icd Δ_icd_for atggaaagtaaagtag ttgttccggcacaagg caagaagatcaccgtg taggctggagctgctt c (SEQ ID NO: 138) Δ_icd_rev ttacatgttttcgatg atcgcgtcaccaaact ctgaacatttcagcat atgaatatcctcctta g (SEQ ID NO: 139) sucA Δ_sucA_for atgcagaacagcgctt tgaaagcctggttgga ctcttcttacctcgtg taggctggagctgctt c (SEQ ID NO: 140) Δ_sucA_rev ttattcgacgttcagc gcgtcattaaccagat cttgttgctgtttcat atgaatatcctcctta g (SEQ ID NO: 141) sucB Δ_sucB_for atgagtagcgtagata ttctggtccctgacct gcctgaatccgtagtg taggctggagctgctt c (SEQ ID NO: 142) Δ_sucB_rev ctacacgtccagcagc agacgcgtcggatctt ccagcaactctttcat atgaatatcctcctta g (SEQ ID NO: 143) frdA Δ_frdA_for gtgcaaacctttcaag ccgatcttgccattgt aggcgccggtggcgtg taggctggagctgctt c (SEQ ID NO: 144) Δ_frdA_rev tcagccattcgccttc tccttcttattggctg cttccgccttatccat atgaatatcctcctta g (SEQ ID NO: 145) frdB Δ_frdB_for atggctgagatgaaaa acctgaaaattgaggt ggtgcgctataacgtg taggctggagctgctt c (SEQ ID NO: 146) Δ_frdB_rev ttagcgtgatttcaaa atcgcaataagaaagt ctttcgaactttccat atgaatatcctcctta g (SEQ ID NO: 147) frdC Δ_frdC_for atgacgactaaacgta aaccgtatgtacggcc aatgacgtccaccgtg taggctggagctgctt c (SEQ ID NO: 148) Δ_frdC_rev ttaccagtacagggca acaaacaggattacga tggtggcaaccaccat atgaatatcctcctta g (SEQ ID NO: 149) frdD Δ_frdD_for atgattaatccaaatc caaagcgttctgacga accggtattctgggtg taggctggagctgctt c (SEQ ID NO: 150) Δ_frdD_rev ttagattgtaacgaca ccaatcagcgtgacaa ctgtcaggatagccat atgaatatcctcctta g (SEQ ID NO: 151) ptsG Δ_ptsG_for atgtttaagaatgcat ttgctaacctgcaaaa ggtcggtaaatcggtg taggctggagctgctt c (SEQ ID NO: 152) Δ_ptsG_rev ttagtggttacggatg tactcatccatctcgg ttttcaggttatccat atgaatatcctcctta g (SEQ ID NO: 153) ptsI Δ_ptsI_for atgatttcaggcattt tagcatccccgggtat cgctttcggtaaagtg taggctggagctgctt c (SEQ ID NO: 154) Δ_ptsI_rev ttagcaaattattttt tcttcaatgaacttgt taaccaacgtcatcat atgaatatcctcctta g (SEQ ID NO: 155)

TABLE 13 Primer pairs used for verification of gene disruptions Deleted Sequence (5′-3′) gene Forward primer Reverse primer K2 for/ cggtgccctgaatgaactgc cagtcatagccgaatagcct k1 rev (SEQ ID NO: 156) (SEQ ID NO: 157) IdhA atacgtgtcccgagcggtag tacacatcccgccatcagca (SEQ ID NO: 158) (SEQ ID NO: 159) adhE gaagtaaacgggaaaatcaa agaagtggcataagaaaacg (SEQ ID NO: 160) (SEQ ID NO: 161) ackA ccattggctgaaaattacgc gttccattgcacggatcacg (SEQ ID NO: 162) (SEQ ID NO: 163) focA_ atgccgtagaagccgccagt tgttggtgcgcagctcgaag pflB (SEQ ID NO: 164) (SEQ ID NO: 165) pta gcaaatctggtttcatcaac tcccttgcacaaaacaaagt (SEQ ID NO: 166) (SEQ ID NO: 167) poxB ggatttggttctcgcataat agcattaacggtagggtcgt (SEQ ID NO: 168) (SEQ ID NO: 169) sad gctgattctcgcgaataaac aaaaacgttcttgcgcgtct (SEQ ID NO: 170) (SEQ ID NO: 171) gabD tctgtttgtcaccaccccgc aagccagcacctggaagcag (SEQ ID NO: 172) (SEQ ID NO: 173) gadA aagagctgccgcaggaggat gccgccctcttaagtcaaat (SEQ ID NO: 174) (SEQ ID NO: 175) gadB ggattttagcaatattcgct cctaatagcaggaagaagac (SEQ ID NO: 176) (SEQ ID NO: 177) gadC gctgaactgttgctggaaga ggcgtgcttttacaactaca (SEQ ID NO: 178) (SEQ ID NO: 179) sfcA tagtaaataacccaaccggc tcagtgagcgcagtgtttta (SEQ ID NO: 180) (SEQ ID NO: 181) maeB attaatggtgagagtttgga tgcttttttttattattcgc (SEQ ID NO: 182) (SEQ ID NO: 183) ppc gctttataaaagacgacgaa gtaacgacaattccttaagg (SEQ ID NO: 184) (SEQ ID NO: 185) pykA tttatatgcccatggtttct atctgttagaggcggatgat (SEQ ID NO: 186) (SEQ ID NO: 187) pykF ctggaacgttaaatctttga ccagtttagtagctttcatt (SEQ ID NO: 188) (SEQ ID NO: 189) iclR gatttgttcaacattaactc tgcgattaacagacaccctt atcgg (SEQ ID NO: 191) (SEQ ID NO: 190) mgsA tctcaggtgctcacagaaca tatggaagaggcgctactgc (SEQ ID NO: 192) (SEQ ID NO: 193) icd cgacctgctgcataaacacc tgaacgctaaggtgattgca (SEQ ID NO: 194) (SEQ ID NO: 195) sucA acgtagacaagagctcgcaa catcacgtacgactgcgtcg (SEQ ID NO: 196) (SEQ ID NO: 197) sucB tgcaactttgtgctgagcaa tatcgcttccgggcattgtc (SEQ ID NO: 198) (SEQ ID NO: 199) frdA aaatcgatctcgtcaaattt aggaaccacaaatcgccata cagac (SEQ ID NO: 201) (SEQ ID NO: 200) frdB gacgtgaagattactacgct agttcaatgctgaaccacac (SEQ ID NO: 202) (SEQ ID NO: 203) frdC tagccgcgaccacggtaaga cagcgcatcacccggaaaca aggag (SEQ ID NO: 205) (SEQ ID NO: 204) frdD atcgtgatcattaacctgat ttaccctgataaattaccgc (SEQ ID NO: 206) (SEQ ID NO: 207) ptsG ccatccgttgaatgagtttt tggtgttaactggcaaaatc (SEQ ID NO: 208) (SEQ ID NO: 209) ptsI gtgacttccaacggcaaaag ccgttggtttgatagcaata (SEQ ID NO: 210) (SEQ ID NO: 211)

TABLE 14 Primers used forgene overexpression. Restriction sites used for cloning into pACT3 are underlined. Gene Primer Linker Sequence Ec_pck Ec_pck_clon_for XmaI tataatcccgggatgc gcgttaacaatggttt gacc (SEQ ID NO: 212) Ec_pck_clon_rev XbaI tataattctagattac agtttcggaccagccg (SEQ ID NO: 213) Ec_ppc Ec_ppc_clon_for XmaI tataatcccgggatga acgaacaatattcc (SEQ ID NO: 214) Ec_ppc_clon_rev XbaI tataattctagattag ccggtattacgcat (SEQ ID NO: 215) Ec_pykA Ec_pykA_clon_for XmaI tataatcccgggatgt ccagaaggcttcgcag aaca (SEQ ID NO: 216) Ec_pykA_clon_rev XbaI tataattctagattac tctaccgttaaaatac (SEQ ID NO: 217) Ec_aceA Ec_aceA_clon_for XmaI tataatcccgggatga aaacccgtacacaaca aatt (SEQ ID NO: 218) Ec_aceA_clon_rev XbaI tataattctagattag aactgcgattcttcag (SEQ ID NO: 219) Ll_pycA Ll_pycA_clon_for XmaI tataatcccgggatga aaaaactactcgtcgc caat (SEQ ID NO: 220) Ll_pycA_clon_rev XbaI tataattctagattaa ttaatttcgattaaca (SEQ ID NO: 221) Ec_galP Ec_galP_clon_for XmaI tataatcccgggatgc ctgacgctaaaaaaca ggggcggt (SEQ ID NO: 222) Ec_galP_clon_rev XbaI tataattctagattaa tcgtgagcgcctattt c (SEQ ID NO: 223)

TABLE 15 Examples of strains constructed for DHB production Strain Relevant genotype MG1655 Wild-type ECE1 pTAC-DHB ECE5 ΔldhA ΔadhE Δpta-ack pTAC-DHB ECE6 ΔldhA ΔadhE Δpta-ack pTAC-DHB pACT3-pck ECE7 ΔldhA ΔadhE Δpta-ack pTAC-DHB pACT3-ppc ECE8 ΔldhA ΔadhE Δpta-ack pTAC-DHB pACT3-pyc ECE10 ΔldhA ΔadhE Δpta-ack ΔpoxB pTAC-DHB ECE11 ΔldhA ΔadhE Δpta-ack ΔpoxB pTAC-DHB pACT3-pck ECE12 ΔldhA ΔadhE Δpta-ack ΔpoxB pTAC-DHB pACT3-ppc ECE13 ΔldhA ΔadhE Δpta-ack ΔpoxB pTAC-DHB pACT3-pyc ECE16 ΔldhA ΔadhE Δpta-ack ΔpoxB Δmae1 ΔsfcA pTAC-DHB pACT3-pck ECE17 ΔldhA ΔadhE Δpta-ack ΔpoxB Δmae1 ΔsfcA pTAC-DHB pACT3-ppc ECE18 ΔldhA ΔadhE Δpta-ack ΔpoxB Δmae1 ΔsfcA pTAC-DHB pACT3-pyc ECE21 ΔldhA ΔadhE Δpta-ack ΔpoxB Δmae1 ΔsfcA ΔfrdBC pTAC-DHB pACT3-pck ECE22 ΔldhA ΔadhE Δpta-ack ΔpoxB Δmae1 ΔsfcA ΔfrdBC pTAC-DHB pACT3-ppc ECE23 ΔldhA ΔadhE Δpta-ack ΔpoxB Δmae1 ΔsfcA ΔfrdBC pTAC-DHB pACT3-pyc ECE30 ΔldhA ΔadhE Δpta-ack ΔpoxB Δmae1 ΔsfcA ΔfrdBC ΔptsG pTAC-DHB pACT3-pck ECE31 ΔldhA ΔadhE Δpta-ack ΔpoxB Δmae1 ΔsfcA ΔfrdBC ΔptsG pTAC-DHB pACT3-ppc ECE32 ΔldhA ΔadhE Δpta-ack ΔpoxB Δmae1 ΔsfcA ΔfrdBC ΔptsG pTAC-DHB pACT3-pyc

Example 12 Production of 2,4-dihydroxybutyric Acid by Fermentation of Glucose

Strains and cultivation conditions: Experiments were carried out with strain E. coli ECE1 co-expressing malate kinase, malate semialdehyde dehydrogenase and DHB dehydrogenase from the plasmid pTAC-DHB (see Example 11), and an isogenic control strain containing only the empty plasmid (i.e. the pTAC backbone without the coding sequences of the above mentioned enzymes). 1 Liter culture medium contained, 20 g glucose, 18 g Na₂HPO₄*12 H₂O, 3 g KH₂PO₄, 0.5 g NaCl, 2 g NH₄Cl, 0.5 g MgSO₄*7 H₂O, 0.015 CaCl₂*2 H₂O, 1 mL of 0.06 mol/L FeCl₃ stock solution prepared in 100 times diluted concentrated HCl, 2 mL of 10 mM thiamine HCl stock solution, 20 g MOPS, 50 μg kanamycin sulphate, and 1 mL of trace element solution (containing per liter: 0.04 g Na₂EDTA*2H₂O, 0.18 g CoCl₂*6 H₂O, ZnSO4*7 H₂O, 0.04 g Na₂MoO4*2 H₂O, 0.01 g H₃BO₃, 0.12 g MnSO₄*H₂O, 0.12 g CuCl₂*H2O.). pH was adjusted to 7 and medium was filter sterilized. All cultivations were carried out at 37° C. on an Infors rotary shaker running at 170 rpm. Overnight cultures (3 mL medium in test tube) were inoculated from glycerol stocks and used to adjust an initial OD₆₀₀ of 0.05 in 100 mL growth cultures cultivated in 500 mL shake flasks. IPTG was added at a concentration of 1 mmol/L when OD₆₀₀ in the growth cultures reached 0.2.

Estimation of DHB concentration by LC-MS/MS analyses:

Culture medium was separated from the cells by centrifugation (Beckmann-Coulter Allegra 21R, Rotor Beckmann S4180, 10 min, 4800 rpm). Clear supernatant was stored at −20° C. until further analysis. DHB content was quantified using an HPLC (Waters) equipped an ACQUITY UPLC BEH column (C18, 1.7 μm, 100×2.1 mm, Waters), coupled to a mass sensitive detector (TQ, Waters, ESI mode, capillary voltage: 2.5 kV, cone voltage 25 V, Extractor voltage: 3V, source temperature: 150° C., desolvation temperature: 450° C., cone gas flow: 50 L/h, desolvation gas flow: 750 L/h). Column temperature was held at 30° C. Mobile phase was a mixture of 88% of a 0.08% tetra-n-butylammonium hydroxide solution, and 12% acetonitrile. Flow rate was fixed at 0.4 mL/min. Injection volume of the samples was 5 μL.

Results:

The DHB content of the culture medium of strain E. coli ECE1 and the control strain was estimated at 8 h and 24 h after inducing the expression of malate kinase, aspartate semialdehyde dehydrogenase, and DHB dehydrogenase by addition of IPTG. As can be seen in Table 16, the strain ECE1 which expressed the DHB pathway enzymes produced significantly higher amounts of DHB than the control strain demonstrating the possibility of the zymotic production of DHB via the metabolic pathway shown in FIG. 1 (i).

TABLE 16 DHB concentration in the culture medium of E coli ECE1 and control strain DHB concentration [mg/L] Time [h] ECE1 control 8 0.80 0 24 2.53 0.24

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The invention claimed is:
 1. A malate kinase that transforms malate into 4-phospho-malate; the sequence of which is selected from the group consisting of SEQ ID NO: 39, 41, 43, and
 45. 2. The malate kinase according to claim 1, wherein the malate kinase is set forth in SEQ ID NO:
 39. 3. A method of producing 2,4-dihydroxybutyric acid (2,4-DHB), comprising: a first step of transforming malate into 4-phospho-malate using a malate kinase of claim 1, a second step of transforming 4-phospho-malate into malate-4-semialdehyde using a malate semialdehyde dehydrogenase selected from the group consisting of SEQ ID NO: 68, 54, 56, 58, 60, 62, 64, 66, and 231, and a third step of transforming malate-4-semialdehyde into 2,4DHB using a DHB dehydrogenase of SEQ ID NO:
 81. 4. The method of claim 3, wherein the malate kinase is set forth in SEQ ID NO:
 39. 5. The method according to claim 3, wherein the malate semialdehyde dehydrogenase is set forth in SEQ ID NO:
 66. 6. The method of claim 3, wherein the malate semialdehyde dehydrogenase is set forth in SEQ ID NO:
 68. 7. An isolated nucleic acid sequence encoding a malate kinase according to claim
 1. 8. The isolated nucleic acid of claim 7, the sequence of which being selected from the group consisting of SEQ ID NO: 40, 42, and
 44. 9. An expression vector comprising the nucleic acid according to claim
 7. 10. A host microorganism expressing a malate kinase; wherein the host microorganism is transformed with the expression vector of claim
 9. 11. The host microorganism according to claim 10, wherein the host microorganism is a bacterium, a yeast, or a fungus.
 12. The host microorganism of claim 11, wherein the host microorganism is selected from the group consisting of Escherichia coli, Saccharomyces cerevisiae, Corynebacterium glutamicum, Zygosaccharomyces rouxii, and Aspergillus flavus.
 13. A chimeric gene comprising at least, in the direction of transcription, functionally linked: a promoter regulatory sequence which is functional in a host organism, one or more isolated nucleic acid sequence encoding a malate kinase according to claim 1, and a terminator regulatory sequence which is functional in the host organism.
 14. The chimeric gene of claim 13 comprising, in the direction of transcription, functionally linked: a promoter regulatory sequence which is functional in a host organism, a nucleic acid sequence encoding a malate semialdehyde dehydrogenase that transforms 4-phospho-malate into malate-4-semialdehyde, that has an enzyme that is an aspartate semialdehyde dehydrogenase, and the sequence being selected from the group consisting of SEQ ID NO: 54, 56, 58, 60, 62, 64, 66, and 231, and a terminator regulatory sequence that is functional in the host organism.
 15. The chimeric gene of claim 14 having the sequence of SEQ ID NO:
 229. 16. An expression vector comprising the chimeric gene according to claim
 15. 17. A host microorganism, which is transformed with the expression vector of claim
 16. 18. A process of production of 2,4-DHB comprising cultivating the host microorganism according to claim
 17. 19. A method of producing 4-phospho-malate, comprising transforming malate by the maltase kinase according to claim
 1. 20. A malate semialdehyde dehydrogenase that transforms 4-phospho-malate into malate-4-semialdehyde; the sequence of which is selected from the group consisting of SEQ ID NO: 54, 56, 58, 60, 62, 64, 66, and
 231. 21. The malate semialdehyde dehydrogenase according to according to claim 20, wherein the malate semialdehyde dehydrogenase is set forth in SEQ ID NO:
 66. 22. An isolated nucleic acid sequence encoding a malate semialdehyde dehydrogenase according to claim
 20. 23. The isolated nucleic acid of claim 22, wherein the nucleic acid has a sequence selected from the group consisting of SEQ ID NO: 55, 57, 59, 61, 63, 65, and
 230. 24. An expression vector comprising the nucleic acid according to claim
 22. 25. A host microorganism expressing a malate semialdehyde dehydrogenase; wherein the host microorganism is transformed with the expression vector of claim
 24. 26. The host microorganism according to claim 25, wherein the host microorganism is a bacterium, a yeast, or a fungus.
 27. The host microorganism of claim 26, wherein the host microorganism is selected from the group consisting of Escherichia coli, Saccharomyces cerevisiae, Corynebacterium glutamicum, Zygosaccharomyces rouxii, and Aspergillus flavus.
 28. A process of production of malate-4-semialdehyde comprising: cultivating the host microorganism according to claim 25 and expressing a malate semialdehyde dehydrogenase, and transforming 4-phospho-malate into malate-4-semialdehyde using the malate semialdehyde dehydrogenase.
 29. A host microorganism, which is transformed with: an expression vector comprising an isolated nucleic acid sequence encoding a malate kinase that transforms malate into 4-phospho-malate, the sequence of which is selected from the group consisting of SEQ ID NO: 39, 41, 43, and 45; the expression vector of claim 24; and an expression vector comprising an isolated nucleic acid sequence encoding a DHB dehydrogenase that transforms malate-4-semialdehyde into 2,4-DHB, where the DHB dehydrogenase is set forth in SEQ ID NO:
 81. 30. A process of production of 2,4-DHB comprising cultivating the host microorganism according to claim
 29. 31. A method of producing malate-4-semialdehyde, comprising transforming 4-phospho-malate by the malate semialdehyde dehydrogenase according to claim
 20. 32. A DHB dehydrogenase that transforms malate-4-semialdehyde into 2,4-DHB; wherein the DHB dehydrogenase is set forth in SEQ ID NO:
 81. 33. An isolated nucleic acid sequence encoding DHB dehydrogenase according to claim
 32. 34. The isolated nucleic acid of claim 33 the sequence of which being SEQ ID NO:
 82. 35. An expression vector comprising the nucleic acid according to claim
 33. 36. A host microorganism expressing a DHB dehydrogenase; wherein the host microorganism is transformed with the expression vector of claim
 35. 37. The host microorganism according to claim 36, wherein the host microorganism is a bacterium, a yeast, or a fungus.
 38. The host microorganism of claim 37, wherein the host microorganism is selected from the group consisting of Escherichia coli, Saccharomyces cerevisiae, Corynebacterium glutamicum, Zygosaccharomyces rouxii, and Aspergillus flavus.
 39. A process of production of 2,4-DHB, comprising: cultivating the host microorganism according to claim 36 and expressing a DHB dehydrogenase, and transforming malate-4-semialdehyde into 2,4-DHB using the DHB dehydrogenase.
 40. The process of production of 2,4-DHB according to claim 39, wherein the host microorganism is cultivated in a medium where malate, pyruvate, succinate, or fumarate is added.
 41. The process of claim 40, wherein the culture medium further comprises another carbon source. 